Numerous systems are known in which a position of a locatable unit is determined. Often, such location systems use location signals communicated between the locatable unit and one or more known-position locators. The locatable unit may only transmit location signals, only receive location signals, or both transmit and receive location signals.
Location systems have been configured that use ultrasonic location signals, infrared location signals, and electromagnetic location signals. Ultrasonic and infrared systems are most often used when locations are determined within small monitored areas. However, even when used over small areas ultrasonic and infrared systems are unreliable. For example, ultrasonic signal receivers are unacceptably prone to false alarms while infrared signal receivers are unacceptably prone to failures in detecting valid alarm signals. Furthermore, if ultrasonic or infrared systems are expanded to cover medium size or larger areas, the system becomes unacceptably costly due to the large number of known-position locators required to extend the detection area.
Electromagnetic systems are better suited to determining locations over large areas. Two techniques are conventionally used to determine a locatable unit's position. In accordance with multiangulation location systems, the location signals are used to determine azimuths between the locatable unit and each of multiple known-position locators. Using trigonometry, these azimuths are processed into a position for the locatable unit relative to the known-position locators. Unfortunately, the determination of azimuths is a complex operation that requires costly equipment.
Multilateration systems are typically less complex and costly than multiangulation systems because they do not determine angles. In accordance with multilateration systems, the location signals are used to determine propagation delays between the locatable unit and each of multiple known-position locators. The location signals propagate between the locatable unit and known-position locators at constant speed. Due to differences in spacing between the locatable unit and various known-position locators, these propagation delays may be processed into a position for the locatable unit relative to the known-position locators.
Conventional electromagnetic location systems are highly sensitive to multipath propagation. Multipathing results when the location signals reach a receiver at either a locatable unit or known-position locator by an indirect or reflected path, and often by two or more paths. In both multiangulation and multilateration systems, the multipath or reflected signals corrupt the locations determined by the system. Consequently, such systems are typically used over wide open spaces where the multipathing phenomenon is minimized.
On the other hand, a need exists for a location system that reliably operates over small and medium size areas. Such areas may include a building, a campus consisting of several buildings and open areas, or an entire urban area. The structures in these small and medium size areas present a severe multipathing problem for a location system that relys upon electromagnetic signals. Consequently, conventional electromagnetic systems exhibit poor performance in these situations.